A seat belt in a vehicle may be equipped with “load limiting” features. During a collision, a retractor of the seat belt may lock webbing of the seat belt from further extension from the retractor, but load-limiting features permit some additional limited extension of the webbing when the force exerted on the webbing exceeds a load-limiting threshold. This additional extension of the webbing from the retractor limits the load applied by the webbing to the chest of an occupant to reduce the chest deflection.
Load limiting may be achieved through use of a torsion bar. One end of the torsion bar may be connected to a spool around which the webbing is wound in the retractor. In the event of a collision, the other end of the torsion bar may be fixed. As an occupant exerts a force tending to extract the webbing from the retractor, the spool applies a twisting force to the torsion bar. The torsion bar yields, that is, deforms, at a known threshold force. The force imparted to an occupant through the seat belt is thus limited by the threshold force of the torsion bar.